This invention relates to tape control apparatus and, more particularly, to such apparatus for controlling the speed, direction and tension of a tape member which is transported bidirectionally between a supply and a take-up reel.
The use of a tape member, such as paper tape, magnetic tape, or the like, has long been used for recording and storing information thereon such that the stored information can be reproduced and utilized at a subsequent time. This information, particularly when recorded on magnetic tape, has been in the form of analog signals representing, for example, audio information, video information or telemetered information, and also in the form of digital signals having particular use in data processing techniques, but also having application to the recording and reproducing of audio or video information. In many of the foregoing uses, such as when video information is recorded on magnetic tape or when magnetic tape is used in a data processing environment, it is necessary to provide accurate control over the direction and speed at which the tape is transported. For example, in a magnetic video recording and/or reproducing device, such as a video tape recorder (VTR), the tape is helically wrapped about at least a portion of a guide drum to enable one or more transducers which are rotatably supported within the drum to scan successive, parallel slant tracks across the tape. Desirably, video signals which are recorded on the tape can be reproduced by the VTR in a normal mode, in a slow-motion mode, in a quick-motion mode, in a still (or stop-action) mode or in a reverse mode. These various modes depend upon the speed and direction at which the tape moves across the guide drum. In each of these modes of operation, the tape should be driven at a uniform speed so as to avoid jitter and other undesirable defects in the video picture which is reproduced from the signals on the tape. Also, if the VTR is selectively operated so as to change the tape speed, such as for rewinding or fast-forwarding, the tape transport apparatus should be properly controlled to prevent the occurrence of a run-away condition, or the occurrence of undesired unwinding of tape, or the bursting or tearing of tape.
In a typical VTR, the magnetic tape normally is transported between a pair of reels such that tape is payed out from the supply reel and wound up, or accumulated, on the take-up reel. This reel-to-reel transport may be present either in the so-called open reel machine or in video cassette or cartridge machines wherein both reels are provided in a single, easily manipulatable housing. In such reel-to-reel transport systems, the tape should be transported with a substantially constant tension. In general, this is achieved by applying a take-up torque to the take-up reel and a counter torque to the supply reel, these opposite torques tending to provide a back-tension to the transported tape. An attendant problem in providing counter torques is to maintain a constant back-tension when the VTR is operated so as to accelerate or decelerate the tape, or to change the direction of tape transport. For example, if the counter torque applied to the supply reel is too small, tape will rapidly unwind therefrom if the transport speed is decelerated. Conversely, if the counter torque applied to the supply reel is too great, the back-tension on the tape may be high enough to damage the tape if its transport speed is accelerated.
One factor affecting the provision of a constant tape tension is the change in the effective diamters of the tape which is wound on the supply and take-up reels. Since these effective diameters are changing as the tape is transported, the inertia of the respective reels likewise is changing, thereby influencing the forward and counter torques which must be applied thereto. Thus, it is desirable to detect the effective reel diameters of the supply and take-up reels so that the torques which are applied thereto can be modified accordingly. While such reel diameters can be detected by using a mechanical follower in combination with a photosensing device, this type of arrangement may not provide the accuracy which is needed in maintaining a constant tape tension.
Another factor which should be taken into account in maintaining a constant tape tension in a tape transport system is friction between the tape and various guide elements in the transport path. Particularly in a VTR, friction exerted on the tape by the guide drum should be considered. At most operating speeds, this friction is independent of the tape speed and may be considered to be a constant. Because of this constant friction, it may be thought that tape tension can be controlled merely by adjusting the forward and counter torques without regard to this friction. However, if this friction changes for some reason, such as if the coefficient of friction of a portion of the tape is less than the remainder thereof, the back-tension on the tape may be too low, and a run-away condition may develop.
In a tape speed controlling device of the type described in U.S. Pat. No. 3,600,654, a forward torque is exerted upon the take-up reel and a counter torque is exerted on the supply reel. This counter torque provides a back-tension on the tape during its forward movement. However, the forward drive voltage which is used to drive the take-up reel merely is divided by a voltage divider circuit to a lower value for imparting a counter torque to the supply reel. If the forward tape speed exceeds a preset value, the voltage dividing ratio is adjusted so as to increase the voltage, and thus the counter torque, applied to the supply reel, thereby increasing the tape tension to reduce the tape speed. In apparatus of this type, neither the tape transport speed nor the tape tension is controlled accurately. Furthermore, the effects of friction in the tape run are not taken into account.
In another type of tape speed control apparatus, such as described in U.S. Pat. No. 4,015,177, there is a more continuous control over the tape transport speed. A signal representing the actual tape speed is compared to a reference signal representing desired tape speed, and any difference therebetween is used to develop voltages which are applied to the supply and take-up reel drive motors. The speed of the tape is maintained constant by differentially controlling the speeds of the reel drive motors. Although the tape speed is dependent upon the difference between the voltages applied to the respective motors, it is not clear whether these individual voltages are differentially related to each other. Furthermore, in this type of system, the effects of tape friction are not taken into account. Nor are the motor control voltages, or torques imparted to the tape reels, adjusted as a function of the changing effective reel diameters. Hence, it is difficult to maintain a desirably constant tension in the transported tape.